Leonard C Suskin's musings on writing, parenthood, and the wonderful world of commercial AV.

Monday, January 23, 2012

Hammers and echoes

It is said that when the tool you have is a hammer, every problem looks like a nail. Last week I was lucky enough to be handed a hammer right before encountering a pile of sticking-up nails. This week's Technology Tuesday is about a part in my personal AV journey when I was given such a hammer right before encountering some nails.

The latest stop in my new AV engineer training is certification with DSPs, or Digital Signal Processors. For the uninitiated, these are audio processing devices that perform a variety of functions in an AV system:

1) Matrix mixing. Sending any one of a number of inputs to any one of a number of outputs.
2) Providing gain control (volume adjustments) to inputs and outputs.
3) Processing, such as level control, filtering, and acoustic echo cancellation

It's in the last part that the devices have their most interesting and powerful features, and where one sees the benefit of digital processing. Acoustic echo cancellation in particular is an important part of any conferencing system. If one connects to another room containing microphones and speakers, the microphones at the far-end location will "hear" the audio from the speakers at that same location. The problem is that if the microphones at your location also hear your speakers, it creates a loop in which the people to whom you're talking will hear a delayed echo of their own voice. An Acoustic Echo Cancellation (AEC) systems compare the signal from your microphones to the incoming signal from the far end and strips their audio from the signal being transmitted. In short, your AEC system prevents your audience from hearing themselves.

What was most interesting to me is that the very next workday after learning exactly how to set this up,I found myself in a new conference room, making a video call to someone who complained that he heard himself, and that my level was too low. This seemed easy. First to adjust the level. The conversation went like this:

"How does that sound?"

"You're too low"

clickclickclick "How about now?"

"Still too low"

clickclickclick "Now?"

"Still low".

At this point I'd nearly maxed out all of my adjustments, and was starting to doubt that I'd actually learned anything. In desperation (and suspecting something wrong), I muted the conference output.

"Now?"

"still low".

"Let me call you back in a few minutes."

The problem, of course, is that the wrong output wire was connected to the outgoing videoconference, so I could adjust until the cows came home without changing anything. I swapped the wire, adjusted the loudness, and made the echo go away.

Moral of the story: Even if life did just give you a hammer, make sure that you're using it on the right nail.